Disease, the effects of aging, or physical trauma resulting in damage to the spine has been treated in many instances by fixation or stabilization of the effected vertebra. The use of plates and screws for fixation and stabilization of the vertebra has been widely accepted as a reliable practice and has proven to be highly successful clinically for both fracture (trauma) and spinal fusion applications. The vertebral plates, which are attached to the anterior portion of the vertebral bodies of the spinal column by bone screws have some common features such as relatively planar body profiles that define multiple holes or slots through which the screws fit and are threaded into the bone. Innovations over time have been primarily directed to improving the dependable security of the bone screws to the plate and as such have resulted in virtually complete rigidity of the screw, bone plate, bone connection. Various means have been used to prevent the screws from becoming loose or detached from their necessary secured or locked attachment to the vertebral plate. Among the differences between the conventionally used plates and screws is the manner in which the screws are locked into place in the hole or slot of the plate after the screws have been secured to the bone.
Early plate designs consisted of bone plates having holes through which screws were passed and secured into the bone. These plates had no special provision for attaching the screws to the plate and as such were susceptible to having the screws back out of the plate over time. There have been clinically reported instances of screws backing out of these type plates with resulting surgical complications. Due to the potential and actual unreliable performance of such plates, the need for secure fixation of the screw to the plate as well as to the bone is now considered a basic requirement for vertebral plates.
One approach to prevent such screw backing out has been to provide features in and on the plate which are specifically designed to hold the screw in position once the screw is inserted through the plate and screwed into the bone. One design option is a cover plate as indicated in U.S. Pat. No. 7,137,984 by Michelson which typically adds thickness to the plate by design. A thicker plate creates more post-operative issues for the patient especially in the cervical spine where the esophagus can easily become aggravated and damaged by the thicker plate designs. Another direction taken in this effort has been to design plates that incorporate or attach individual retaining rings or snap features associated with each plate hole configuration to hold the inserted screw in place relative to the plate, such as that depicted in US2010/0241174 by Robinson. While designs such as US2010/0241174 by Robinson provide improved screw security over a plate with no retention feature with reduced thickness compared to cover designs such as Michelson, applicants have determined that there is room for improvement with respect to screw security in the Robinson design. Therefore a need exists for an optimal way to reliably retain the screw, while not increasing plate thickness.